(Note: some of this is information copied directly from http://adventurealan.com/iphone4gps.htm, other is my own information)
The ATT iPhone 4 (and other SIM based iPhones) is a viable tool to use as a GPS and mapping device for backpacking trips worldwide. In many ways it is superior to traditional mapping GPS units like the Garmin Oregon. How? In general, the iPhone 4 is a lighter item, has more abilities (notepad, phone, whatever app), and has at least FOUR times the resolution (detail) of standard gps. You already own one (right?!), so you just saved $400! There are also dozens of products out there for extending battery life, recharging, etc. To top it off, additional maps are often much cheaper (or free) compared to maps designed for gps units. Note: There are some companies that make amazing maps for GPS units that are probably not yet available on an iPhone.
Oh yeah, almost forgot ... you also have the ability to make emergency calls / texts or contact a family member/packer if you can manage phone signal!
This post will tell you how to get the most of your iPhone as a backpacking GPS.For gps and mapping purposes, the Verizon version of the iPhone 4 is identical to the ATT version except it does not have the same ability to disable the phone, and therefore managing battery life is problematic. See the Battery Conservation section for more information.
The method for using the iPhone for backpacking is to use apps that let you preload the appropriate maps and data files over WiFi. When you are out on the trail without cell service, you can use the preloaded maps along with the iPhone’s GPS. If you already own an iPhone, then the cost of using it as a GPS/Mapping device for backpacking is very reasonable.
There are hundreds of iPhone mapping/GPS applications to choose from, and there are seemingly new ones everyday. You should probably buy a couple applications; they are not expensive, and the map content is usually free (unlike Garmin, where you pay substantially for map content).
Some suggested Apps:
MotionX - The latest version of this app cost me $1 and does exactly what I needed it to do: download maps while on wifi and use them in the field. The best part? You can download those amazing new BING satellite imagery, as well as their topo versions. There are several other map types. Unfortunately, there are no public / private land options available, but for $1 it is hard to beat.
Gaia GPS - This app lets you store topo maps and satellite imagery for offline use; import and export gpx/kml files; create waypoints, record tracks, and view trip statistics. This is the mapping/GPS app we recommend as the primary tool for use worldwide, to be supplemented by other special purpose tools noted below. (GPS Kit, and ViewRanger are alternatives to Gaia GPS that some people might like more, based on nuanced differences in features set. In our opinion these three apps are inferior; for more info see the list of the all of the apps we reviewed.)
Tips on Using the iPhone
Regardless of which app you choose, there are a few considerations for using an iPhone in the backcountry.
Data - When you have a WIFI connection, download the map content (and trail data if you have it) that you’ll need while hiking. Your iPhone can get GPS signal in the woods, but you will be unlikely to reach the Internet for data.
Battery Life - Manage your battery life. Day hikers can get away without taking special steps to tend the battery life, but for multi-day trips, you must tend to these things. This article closes with detailed instructions on how to maximize battery life, because we think that’s the true crux of the issue.
Protect Your Phone - If you are going to be hiking with an iPhone, you should probably have ways to protect it from the elements. There are a number of cases to help protect from bumps and scratches, and a zip lock bag (Heavy Duty Freezer Pint size) or Aloksak (4.5x7") is a simple and cheap way to make sure rain or a wild river encounter doesn’t cost you several hundred dollars.
GPS Accuracy - We have not yet compared the GPS accuracy to Garmin in a variety of conditions. When using the iPhone4 in shallow grade mountains or flat landscapes, with minor tree cover, the accuracy has consistently been within 10 meters. When using it in steep walled canyons, the fix is not as accurate as Garmin, 50 to 100 or more meters off in a deep canyon. We leave it to the reader make their own determination if this level of accuracy is sufficient. We have not tested under heavy tree cover.
Battery Conservation Settings for field use of your iPhone
It's important to follow these guidelines to conserve your iPhone battery and get the maximum battery life in the field, particularly for extended trips. Our research ended up focusing a great deal on battery life, and we hope these ideas help:
Make sure you can see your battery level at all times.
Do this so you can get better resolution information than is available with the battery symbol in the upper right corner. Go to Settings->General->Usage->BatteryPercentage and set it to On.
Tweak all the Settings as per Apple’s recommendations http://www.apple.com/batteries/iphone.html - However, don’t turn on Airplane Mode as they suggest, and don’t disable location services (i.e., GPS), since that is the one battery-intensive service that you really do need.
Shut down all extraneous apps - iPhone 4 supports a form of multi-tasking or background processing, and you are not likely to know what’s running in the background. Some apps (but not most) can actually do work in the background and consume power. To be completely sure you don’t have anything consuming power, fully shut down all the apps you aren’t using. It’s not intuitively obvious how to do this. Double-click the home button to display the recently accessed applications. Touch and hold any of the icons in the bottom bar until the icon shakes and the red circle is visible. Tap the red circle to shut down an app. This looks similar to the graphic used for deleting an app, however with this method you are merely shutting it down, not deleting it.
Disable the phone while backpacking.
This is probably the most important part of this post.
When the phone is enabled there is significant background battery drain. Baseline battery drain for ATT iPhone 4, with the phone asleep and all other battery conservation measures in place:
<0.1% per hour (1-2% per day): SIM Inactive.
~0.4% per hour (9-10% per day): SIM Active (signal present).
~1.2% per hour (nearly 30% per day!): SIM Active (no signal).
ATT iPhone 4 (and other SIM based iPhones). To absolutely maximize battery life, deactivate the phone’s SIM with the SIM PIN feature under Settings -> Phone -> SIM PIN (http://support.apple.com/kb/ht1316 - The default is 1111 - call 611 from your phone to get your initial PIN from ATT if this fails). When using this feature, you are asked for the PIN code only after the phone is fully powered down, not after each time the phone has been asleep. In order to turn off the phone, restart your iPhone
Verizon iPhone 4. The Verizon phone uses CDMA instead of SIM card. The only way to disable the phone is to turn Airplane Mode on. But this also disables the GPS. To make Verizon phone battery life viable for backpacking, you would need to keep the phone in airplane mode and temporarily toggle airplane mode off every time you want to get a gps read .
Don’t use the Tracking feature.
Most of the map apps allow you to save the track of where you’ve been, but to do this it must constantly get a GPS read and store that data, which is a steady battery drain. Instead, make sure you’re not in Tracking mode and just get your current location when necessary. In our battery tests we found that Tracking mode consumes ~5% of the battery capacity per hour. If you do all the other battery-conservation steps, then you can use Tracking for a long day hike (~60% of the battery will be used in 12 hours of Tracking), but the Tracking feature is only viable for a multi-day backpacking trip if you have a battery recharge solution.
Be EXTRA cautious about leaving the GPS app in the foreground during sleep.
If Tracking is off and the app is in the background when the iPhone is put to sleep, none of the apps we tested incurred battery drain (except ViewRanger, which was horrible in this respect). However, if Tracking is off and the app is in the foreground when the iPhone is put to sleep, quite a few of the apps we tested incurred a battery drain of between 2 and 7% per hour (including MotionX, GPS Kit, Trail Maps by National Geographic, ViewRanger, and EveryTrail Pro). This is a significant issue and hopefully is a bug that the app’s respective developers will fix soon. Whichever app you choose to use, make sure you check to see if you must manually put the app in the background before putting the device to sleep, and if so, you must be absolutely diligent about doing so.
Beware of Auto-Lock.
Under iPhone->Settings->General you’ll find Auto-Lock. After the user-specified duration of inactivity, the iPhone automatically goes to sleep as a battery conservation measure (since the display itself is a big battery drain). Of the nine GPS apps we tested, four of them did not respect that setting, and the iPhone stayed on indefinitely if the application was in the foreground. Whichever app you choose, make sure you test to see if it prevents Auto-Lock from engaging and adjust your behavior accordingly.
Keep the iPhone warm.
Batteries achieve optimum service life if used at 20°C (68°F) or slightly below. Avoid letting the iPhone overheat in direct sun, and keep it in pocket close to your body in cold conditions. More detailed battery information in this great article.
My previous GPS was a Delorme PN-40 (ps: will sell it to you for $100!) with their TOPO USA maps. I now use my iPhone 4 and MotionX GPS. I am getting battery life of days instead of hours (The Delorme would chew through 4 AAs in about 6 hours), signal acquisition in a quarter of the time as well as better constant signal and better maps to boot. In the past month, I have used my iPhone4 as a gps to navigate to 3 cross country alpine lakes for some great fishing. Last weekend, I took it to the Sierras for a 4 day trek. I turned the device completely off at night and occasionally when we were on good trail, and even used GPS tracking twice during the trip, for 7 miles of the 30 mile hike. By day 4 I still had 60% battery life.